According to the IMD, if after May 10, 60 per cent of the available 14 stations –Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu and Mangalore — report 2.5 mm or more rainfall for two consecutive days, the onset of monsoon over Kerala can be declared on the second day. This is one of the main parameters for declaring the arrival of monsoon.

Besides this factor, the westerly winds must be up to 15,000 feet above main sea level and outgoing long-wave radiation less than 200 wm-2 (watt per square metre) to declare the arrival of monsoon.

All the necessary parameters were met following which the onset of monsoon over Kerala was announced, Mritunjay Mohapatra, Additional Director General, IMD said.